shell: don't crash if a pointer's focus is null
shell: don't crash if a pointer's focus is null

It's possible for a pointer's focus to be null, e.g. because
the focus surface has been bestroyed. Prevent a crash when
that happens and a client takes too long to respond to a ping.

Signed-off-by: Emilio Pozuelo Monfort <74af59c529c33a30889589bc73d19ca9a6fb0e6a@collabora.co.uk>

jonnylamb/weston,Fantu/compositor-spice,Gnurou/weston,Tarnyko/weston-xdg_surface_present,mchalupa/weston,krezovic/weston,xorgy/weston,xorgy/weston,giucam/weston,kwm81/weston,Tarnyko/weston-xdg_surface_present,jonnylamb/weston,Fantu/compositor-spice,sir-murray/weston,udoprog/weston,eyolfson/weston,udoprog/weston,Gnurou/weston,eyolfson/weston,sir-murray/weston,Fantu/compositor-spice,kwm81/weston,krezovic/weston,mchalupa/weston,giucam/weston